Water heating devices are applied in all kind of applications, such as steam irons, electric kettles, hot drink vending machines, etc. A problem with such devices is that scale may form on the heating elements that are in contact with the water.
During operation of for instance a steam generation device, water is supplied to a part of the water infrastructure where it is heated, such as in the (external) boiler of a system iron, as a consequence of which scale may be formed. If the scale is not (periodically) removed clogging up may occur, as a result of which the performance of the steam generation device may decrease and, eventually, the steam generation device may not be fit to be used anymore.
Hard water containing significant amount of Ca2+ and HCO3− (bicarbonate) may form scale (CaCO3) upon temperature increase via the following chemical reaction:Ca(HCO3)2→CaCO3+H2O+CO2 
Especially boiling water will separate scale, the scale will form in the water but also on the heating element itself as it has the highest temperature. In time scale will grow on the heating element and when internal stresses increase it will break loose from the element. Several treatments of water to prevent scaling have been claimed in the literature. A well known method is the use of ion exchangers were Ca2+ is exchanged for Na+ or H+. A second well known method is the use of phosphonate that in small amount is added to the water and inhibits the formation of seeding crystals in the hard water, effectively preventing the growing of crystals and thus the formation of scale.
In the former a cartridge needs to be used with the ion exchange resin inside. After depletion the cartridge needs to be regenerated or replaced with a new one. In the latter case continuously phosphonate needs to be added as the phosphonates have limited stability at pH 7-8.5, the pH of hard water. The continuous addition can be implemented for example by using a hard-pressed tablet that very slowly liberates the phosphonates into the water. This way of working has been used in prior art steam irons. However, chemicals are added into the water which may be a drawback, for instance when water is (also) meant to be potable.
Physical methods to prevent scale formation have also been claimed but these may have a less clear working principle and the efficacy may sometimes even be doubtful in some cases. For example the use of (electro) magnets placed on water tubing for scale prevention is an example of a poorly understood and non-repeatable scale prevention method.
Also WO2012011026 and WO2012011051 describe methods to prevent scaling.